Physics Of Fluids-Classical Physics-Handouts, Lecture notes of Classical Physics

Download Physics Of Fluids-Classical Physics-Handouts and more Lecture notes Classical Physics in PDF only on Docsity! PHYSICS –PHY101 VU © Copyright Virtual University of Pakistan 50 2 Lγ LwT soap film Summary of Lecture 18 – PHYSICS OF FLUIDS 1. A fluid is matter that has no definite shape and adjusts to the container that it is placed in. Gases and liquids are both fluids. All fluids are made of molecules. Every molecules attracts other molecules around it. 2. Liquids exhibit surface tension. A liquid has the property that its free surface tends to contract to minimum possible area and is therefore in a state of tension. The molecules of the liquid exerts attractive forces on each other, which is called cohesive forces. Deep inside a liquid, a molecule is surrounded by other molecules in all directions. Therefore there is no net force on it. At the surface, a molecule is surrounded by only half as many molecules of the liquid, because there are no molecules above the surface. 3. The force of contraction is at right angles to an imaginary line of unit length, tangential to the surface of a liquid, is called its surface tension: . Here is the force exerted by the "skin" of the F F L γ = liquid. The SI unit of the surface tension is N/m. 4. Quantitative measurement of surface tension: let be the weight of the sliding wire, force with which you pull the wire downward. Obviously w T = , net downward force. Since film has both front and back surfaces, the force acts along a total length of 2 . The surface tension in the film is defined as, 2 . 2 Hence, th T w F F FL F L L γ γ + = = = ⇒ = e surface tension is . 2 5. Let's ask how much work is done when we stretch the skin of a liquid. If we move the sliding wire through a displacement , the work done is . Now is a conser w T L x F x F γ += Δ Δ vative force, so there is potential energy where is the length of the surface layer change in area of the surface. Thus . So we see that surface tension is the U F x L x L UL x A A γ γ Δ = Δ = Δ Δ Δ = Δ = = Δ surface potential energy per unit area ! docsity.com PHYSICS –PHY101 VU © Copyright Virtual University of Pakistan 51 1 2 1 1vL t= 2 2vL t= 6.When liquids come into contact with a solid surface, the liquid's molecules are attracted by the solid's molecules (called "adhesive" forces). If these adhesive forces are stronger than the cohesive forces, the liquid's molecules are pulled towards the solid surface and liquid surface becomes curved inward (e.g. water in a narrow tube). On the other hand, if cohesive forces are stronger the surface becomes curved outwards (e.g. with mercury instead). This also explains why certain liquids spreadwhen placed on the solid surface and wet it (e.g., water on glass) while others do not spread but form globules (e.g., mercury on glass). 0p p 7. The surface tension causes a pressure difference between the inside and outside of a soap bubble or a liquid drop. A soap bubble consists of two spherical surface films with a thin layer of 0 liquid between them. Let pressure exerted by the upper half, and external pressure. force exerted due to surface tension is 2(2 ) (the "2" is for two surfaces). In equilibrium the ne p p rπ γ = = ∴ 2 2 0 0 0 t forces must be equal: 2(2 ) . So the excess pressure is 4 . For a liquid drop, the difference is that there is only 2 surface and so, excess pressure . r p r r p p p r p p r π π γ π γ γ = + − = = − = 1 2 1 2 1 1 1 1 1 2 8. From the fact that liquids are incompressible, equal volumes of liquid must flow in both sections in time t, i.e. V / V / V V . But you can see that V v and similarly that V t t A L A t A = ⇒ = = = = 2 2 2 2 1 1 2 2v . Hence v v . This means that liquid will flow faster when the tube is narrower, and slower where it is wider. L A t A A= = docsity.com